A sleeper vehicle is a car, SUV, or truck designed to look ordinary, stock, or slow on the outside, while hiding a high-performance engine and upgraded mechanics underneath. They are built to blend into traffic and avoid attention, offering immense speed and power that surprises other drivers. 

Key Characteristics of a Sleeper Vehicle
Unassuming Exterior: They lack flashy spoilers, large vents, or loud graphics, often appearing as standard, practical, or commuter-focused, according to Top Speed and Capital One.
High Performance: Despite their dull appearance, they feature powerful engines (often V8s or turbocharged engines) capable of rapid acceleration.
Practicality: Many sleepers are functional sedans, wagons, or SUVs, allowing for daily use while providing sports-car-level speed, say Capital One and CarParts.com.
"Wolf in Sheep’s Clothing": As noted by Family Handyman and Wikipedia, this term highlights the contrast between the vehicle's appearance and its power